Process for polymerizing monomers in fluidized beds

ABSTRACT

The invention is directed toward polymerizing or copolymerizing alpha-olefins either alone or in combination with one or more other alpha-olefins in a gas phase reactor having a fluidized bed and a fluidizing medium such that the fluidizing medium entering the reactor comprises a gas and a liquid phase.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation-in-part of U.S. Ser. No.07/854,041 filed Mar. 19, 1992, now abandoned.

FIELD OF THE INVENTION

This present invention relates to a process for the gas phasepolymerization of olefins in fluidized bed reactors. The presentinvention allows for substantial savings in energy and capital cost bysignificantly increasing the polymer production rate capacity of a givensized reactor.

BACKGROUND OF THE INVENTION

The discovery of the process for the production of polymers in fluidizedbeds has provided a means for the production of a diverse array ofpolymers. Using a gas fluidized bed polymerization process substantiallyreduces the energy requirements as compared to other processes and mostimportantly reduces the capital investment required to run such aprocess.

Gas fluidized bed polymerization plants generally employ a continuouscycle. In one part of the cycle, in a reactor a cycling gas stream isheated by the heat of polymerization. This heat is removed in anotherpart of the cycle by a cooling system external to the reactor.

Generally in a gas fluidized bed process for producing polymers frommonomers a gaseous stream containing one or more monomers iscontinuously passed through a fluidized bed under reactive conditions inthe presence of a catalyst. This gaseous stream is withdrawn from thefluidized bed and recycled back into the reactor. Simultaneously,polymer product is withdrawn from the reactor and new monomer is addedto replace the polymerized monomer.

It is important to remove heat generated by the reaction in order tomaintain the temperature of the gaseous stream inside the reactor at atemperature below the polymer and catalyst degradation temperatures.Further, it is important to prevent agglomeration or formation of chunksof polymer that cannot be removed as product. This is accomplishedthrough controlling the temperature of the gaseous stream in thereaction bed to a temperature below the fusion or sticking temperatureof the polymer particles produced during the polymerization reaction.Thus, it is understood that the amount of polymer produced in afluidized bed polymerization process is directly related to the amountof heat that can be withdrawn from a reaction zone in a fluidized bedwithin the reactor.

Conventionally, heat has been removed from the gaseous recycle stream bycooling the stream outside the reactor. A requirement of a fluidized bedprocess is that the velocity of the gaseous recycle stream be sufficientto maintain the fluidized bed in a fluidized state. In a conventionalfluidized bed reactor, the amount of fluid circulated to remove the heatof polymerization is greater than the amount of fluid required forsupport of the fluidized bed and for adequate mixing of the solids inthe fluidized bed. However, to prevent excessive entrainment of solidsin a gaseous stream withdrawn from the fluidized bed, the velocity ofthe gaseous stream must be regulated. Also, in a steady state fluidizedbed polymerization process wherein the heat generated by thepolymerization reaction is proportional to the rate of polymerproduction, the heat generated is equal to the heat absorbed by thegaseous stream and lost by other means such that the bed temperatureremains constant.

For a time, it was thought that the temperature of the gaseous streamexternal to the reactor otherwise known as the recycle streamtemperature could not be decreased below the dew point of the recyclestream. The dew point of the recycle stream is that temperature at whichliquid condensate begins to form in the gaseous recycle stream. It wasbelieved that introducing a liquid into a gas phase recycle stream in afluidized bed polymerization process would inevitably result in pluggingof the recycle stream lines, the heat exchanger, the area below thefluidized bed or the gas distributor plate. As a consequence ofoperating at a temperature above the dew point of the recycle stream toavoid the problems associated with liquid being in the gaseous recyclestream, production rates in commercial reactors could not besignificantly increased without enlarging reactor diameters.

In the past there was concern that excessive amounts of liquid in therecycle stream would disrupt the fluidization process to the extent thatthe fluidized bed would collapse resulting in the sintering of solidpolymer particles into a solid mass causing the reactor to shut down.This widely held belief to avoid liquid in the recycle stream can beseen from the following: U.S. Pat. Nos. 3,922,322, 4,035,560, 4,359,561,5,028,670 and European Patent Application Nos. 0 050 477, 0 100 879.

Contrary to this belief, it has been demonstrated, as disclosed byJenkins, III, et al. in U.S. Pat. No. 4,543,399 and related U.S. Pat.No. 4,588,790 that a recycle stream can be cooled to a temperature belowthe dew point in a fluidized bed polymerization process resulting incondensing a portion of the recycle stream. The disclosures of these twoJenkins, III, patents are incorporated herein by reference. Theresulting stream containing entrained liquid is then returned to thereactor without the aforementioned agglomeration and/or pluggingphenomena believed to occur when a liquid is introduced into a fluidizedbed polymerization process. This process of purposefully introducing aliquid into a recycle stream is known in the industry as a "condensedmode" operation in a gas phase polymerization process.

The above-mentioned U.S. patents to Jenkins, III, et al. disclose thatwhen a recycle stream temperature is lowered to a point below its dewpoint in "condensed mode" operation, an increase in polymer productionis possible, as compared to production in a non-condensing mode becauseof increased cooling capacity. Also, Jenkins, III, et al. found that asubstantial increase in space time yield, the amount of polymerproduction in a given reactor volume, can be achieved by operating in"condensed mode" with little or no change in product properties.

The liquid phase of the two-phase gas/liquid recycle stream mixture in"condensed mode" remains entrained or suspended in the gas phase of themixture. The cooling of the recycle stream to produce this two-phasemixture results in a liquid/vapor equilibrium. Vaporization of theliquid occurs only when heat is added or pressure is reduced. Theincrease in space time yields achieved by Jenkins, III, et al. are theresult of this increased cooling capacity of the recycle stream which,in turn, is due both to the greater temperature differential between theentering recycle stream and the fluidized bed temperature and to thevaporization of condensed liquid entrained in the recycle stream.

Jenkins, et al. illustrate the difficulty and complexity of control ingeneral and of trying to extend the stable operating zone to optimizethe space time yield in a gas phase reactor.

In Jenkins, et al. the recycle gas is cooled and added to the reactor ata temperature below the dew point so that condensed fluids evaporateinside the reactor. The cooling capacity of the recycle gas can beincreased further while at a given temperature of the cooling heattransfer medium. One option described is to add non-polymerizingmaterials (isopentane) to increase the dew point. Because of greatercooling more heat can be removed and therefore higher space time yieldsare said to be possible. Jenkins, et al. recommends not exceeding 20weight percent, preferably 2 to 12 weight percent, of condensed liquidin the recycle gas. Some of the potential hazards disclosed include theformation of "mud", maintaining a sufficiently high recycle gas speed oravoiding accumulation of liquid on a distributor plate. Jenkins, et al.is silent on where upper limits for non-polymerizable or polymerizablecondensable materials lie and the question of how to optimize the spacetime yield using condensed mode.

A gas fluidized bed reactor may be controlled to give the desired meltindex and density for the polymer at an optimum production. Great careis generally taken to avoid conditions which can lead to formation ofchunks or sheets or, in a worse case, an unstable fluidized bed whichcollapses, or causes polymer particles to fuse together. The control ofa fluidized bed therefore has to be exercised to reduce chunking andsheeting and to prevent bed collapse or a need to terminate the reactionand shut down the reactor. This is the reason why commercial scalereactors are designed to operate well within proven stable operatingzones, and why the reactors are used in a carefully circumscribedfashion.

Even within the constraints of conventional, safe operation, control iscomplex adding further to the difficulty and uncertainty ofexperimentation if one wishes to find new and improved operatingconditions.

There are target values, determined by the polymer and the catalyst, forthe operating temperature, the ratio of comonomer(s) to monomer and theratio of hydrogen to monomer. The reactor and cooling system arecontained within pressure vessels. Their contents are monitored, withoutunduly interfering with fluidization by measuring amongst others (1) thepressure at the top; (2) pressure differential at various heights alongthe bed, (3) temperature upstream of the bed; (4) temperature in thefluidized bed and temperature downstream of the bed as well as (5) thegas composition and (6) gas flow rate. These measurements are used tocontrol the catalyst addition, the monomer partial pressure and velocityof the recycle gas amongst others. Polymer removal is constrained incertain cases by the settled bulk density (non-fluidized) or thefluidized bulk density depending on plant design and these too must bewatched as well as the ash level in the polymer. The plant is a closedsystem. In operation changes in the process of one or more of themeasured values lead to consequential changes elsewhere. In the designof plant the optimization of capacity depends on the most restrictingelement in the overall design.

There is no generally accepted view as to what causes chunking orsheeting. Obviously some fusing together of the polymer particles isinvolved, possibly because of insufficient heat transfer caused byinadequate fluidization in the fluidized bed. However, no clearcorrelations have thus far been found between individual settings andmeasurements and the occurrence of chunking and sheeting. The entiretyof the measured values and controls is used therefore conventionally tostay within known, safe operating areas for a given plant design.

Large scale gas phase plants are expensive and highly productive. Risksassociated with experimentation in such plants are high because downtimeis costly. Therefore it is difficult to explore design and operatingboundaries experimentally in view of the costs and risks.

It will be desirable to provide a method of determining a stableoperating condition for gas fluidized bed polymerization to facilitateoptimum design of the plant and the determination of desirable processconditions in a given plant design. It would also be desirable toprovide a gas fluidized bed polymerization process giving a maximumreactor productivity.

It is hence amongst the aims of the invention to help determine stableoperating zones for a gas fluidized bed process and plant design, tofind criteria for running a process safely with low risk of malfunctionand at the same time high reactor productivities, and/or to avoid anyconstriction in the overall plant capacity due to the reactorproductivity.

SUMMARY OF THE INVENTION

This invention relates to a process for polymerizing alpha-olefins in agas phase reactor at significantly higher production rates than hereforeenvisaged. The invention is directed toward a process for polymerizingalpha-olefins in a gas phase reactor having a fluidized bed and afluidizing medium where the level of liquid in the fluidizing medium isgreater than 20 weight percent based on the total weight of thefluidizing medium.

The invention is also directed toward a process for polymerizingalphaolefins in a gas phase reactor having a fluidized bed and afluidizing medium such that the enthalpy change of the fluidizing mediumexiting and entering the reactor is greater than 40 Btu/lb.

The invention further provides a process for polymerizing alpha-olefinsin a gas phase reactor at a production rate greater than about 500lb/hr-ft².

This invention in another embodiment relates to a method for determiningstable operating conditions of a gas phase fluidized bed polymerizationreactor by identifying a property useful to determine stability of afluidized bed and controlling the composition of a fluidizing medium orrecycle stream to establish a range of values for the property tomaintain the stable operating condition.

The invention in another embodiment is also directed toward a processfor controlling a gas phase fluidized bed polymerization reactor bymonitoring a condition of the reactor indicative of an onset of afailure condition and controlling the composition of a fluidizing mediumor recycle stream in response to the onset to avoid the occurrence ofthe failure condition. In a preferred embodiment a ratio of a fluidizedbulk density (FBD) to settled bulk density (SBD) is monitored. Thisratio is maintained above about 0.59.

The invention still further provides in another embodiment a method ofdetermining stable operating conditions of a gas fluidized bedpolymerization reactor operating in condensed mode which comprisesobserving fluidized bulk density changes in the reactor associated withchanges in the composition of the fluidizing medium; and increasing thecooling capacity of the recycle stream without exceeding the level atwhich a reduction in the fluidized bulk density becomes irreversible. Asa general rule a reduction in the ratio of FBD to SBD to less than 0.59may involve risk of fluidized bed disruption and is to be avoided.

In another embodiment of the invention there is provided a gas fluidizedbed polymerization process for the polymerization of polymer by passinga gaseous stream comprising monomer through a fluidized bed reactor inthe presence of a catalyst under reactive conditions, to producepolymeric product and a stream comprising unreacted monomer gases,compressing and cooling said stream, mixing said stream with feedcomponents and returning a gas phase and a liquid phase to said reactor,the improvement which comprises cooling said stream such that the liquidphase is greater than 15 percent preferably greater than 20 percent byweight of the total weight of the returned stream and the streamcomposition is such that the ratio of fluidized bulk density to settledbulk density is maintained above at least about 0.59.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, features, and advantages of this invention willbecome clearer and more fully understood when the following detaileddescription is read in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic illustration of the preferred embodiment of thereactor used in the practice of the improved gas fluidized bedpolymerization process for the production of polymers of this presentinvention.

FIG. 2 is a plot of isopentane mole percent and fluidized bulk densityof Table 1.

FIG. 3 is a plot of isopentane mole percent and fluidized bulk densityof Table 2.

FIG. 4 is a plot comparing FIG. 2 and FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In the description which follows, like parts are indicated throughoutthe specification and drawing with the same reference numerals,respectively. The drawing is not necessarily to scale, and certain partshave been exaggerated to better illustrate the improved process of thisinvention.

This invention is not limited to any particular type or kind ofpolymerization or copolymerization reaction but is particularly wellsuited to the polymerization reactions involving the polymerization ofone or more of the monomers, for example olefin monomers of ethylene,propylene, butene-1, pentene-1, 4-methylpentene-1, hexene-1, octene-1and styrene. Other monomers can include polar vinyl, conjugated andnon-conjugated dienes, acetylene and aldehyde monomers.

The catalysts employed in the improved process can include coordinatedanionic catalysts, cationic catalysts, free-radical catalysts, anioniccatalysts and include a transition metal component or a metallocenecomponent including single or multiple cyclopentadienyl componentsreacted with either a metal alkyl or alkoxy component or an ioniccompound component. These catalysts include partially and fullyactivated precursor compositions and those catalysts modified byprepolymerization or encapsulation.

Although, as previously stated, this present invention is not limited toany specific type of polymerization reaction, the following discussionof the operation of the improved process is directed to the gas phasepolymerization of the olefin-type monomers, for example polyethylene,where this present invention has been found to be particularlyadvantageous. A significant increase in the reactor productivity ispossible without an adverse effect on the product quality or properties.

To achieve higher cooling capacities, and hence higher reactorproductivity it may be desirable to raise the dew point of the recyclestream to permit a larger increase in the heat removed from thefluidized bed. For the purposes of this application the terms recyclestream and fluidizing medium are interchangeable. The dew point of therecycle stream can be increased by increasing the operating pressure ofthe reaction/recycle system and/or increasing the percentage ofcondensable fluids and decreasing the percentage of non-condensablegases in the recycle stream in the manner disclosed by Jenkins, et al.U.S. Pat. Nos. 4,588,790 and 4,543,399. The condensable fluid may beinert to the catalyst, reactants and the polymer product produced; itmay also include comonomers. The condensable fluid can be introducedinto the reaction/recycle system at any point in the system, as will belater illustrated from FIG. 1. For the purposes of this patentapplication the term condensable fluids include saturated or unsaturatedhydrocarbons. Examples of suitable inert condensable fluids are readilyvolatile liquid hydrocarbons, which may be selected from saturatedhydrocarbons containing from 2 to 8 carbon atoms. Some suitablesaturated hydrocarbons are propane, n-butane, isobutane, n-pentane,isopentane, neopentane, n-hexane, isohexane, and other saturated C₆hydrocarbons, n-heptane, n-octane and other saturated C₇ and C₈hydrocarbons or mixtures thereof. The preferred inert condensablehydrocarbons are C₅ and C₆ saturated hydrocarbons. The condensablefluids may also include polymerizable condensable comonomers such asolefins, diolefins or mixtures thereof including some of theaforementioned monomers which may be partially or entirely incorporatedin the polymer product.

In practicing the invention, the amount of gas in the recycle stream andthe velocity of the recycle stream should be maintained at levelssufficient to keep the liquid phase of the mixture suspended in the gasphase until the recycle stream enters the fluidized bed, so that liquiddoes not accumulate in the bottom head of the reactor below thedistributor plate. The velocity of the recycle stream must also be highenough to support and mix the fluidized bed within the reactor. It isalso desirable that the liquid entering the fluidized bed be dispersedand vaporized quickly.

Controlling the composition, temperature, pressure and superficialvelocity of the gas in relation to the composition and physicalcharacteristics of the polymer is important in maintaining a viablefluidized bed. A viable fluidized bed or a stable operating condition isdefined as fluidized bed of particles that are suspended and well-mixedin a stable state under reactive conditions without the formation ofsignificant quantities of agglomerates (chunks or sheets) which woulddisrupt the reactor or downstream process operations.

In the one preferred embodiment more than 15 weight percent, preferablygreater than 20 weight percent, of the recycle stream may be condensed,or be in a liquid phase without encountering disruption of thefluidization process provided the safe operating boundaries of thestable operating zones determined with the help of fluidized bed bulkdensity measurements are not exceeded.

During the polymerization process, a minor portion (typically less thanabout 10 percent) of the gaseous stream flowing upward through thefluidized bed reacts. That major portion of the stream that does notreact passes into a region above the fluidized bed called the freeboardzone which may be a velocity reduction zone. In the freeboard zone, thelarger solid polymer particles which are projected above the bed byeruption of gas bubbles through the surface or entrained in the gasstream are allowed to fall back into the fluidized bed. The smallersolid polymer particles, known in the industry as "fines", are withdrawnwith the recycle stream because their terminal settling velocities arelower than the velocity of the recycle stream in the freeboard zone.

The process operating temperature is set or adjusted to a temperaturebelow the fusion or sticking temperature of polymer particles produced.Maintaining this temperature is important to prevent the plugging of thereactor by polymer chunks that grow rapidly if the temperature reacheshigh levels. These chunks of polymer can become too large to bewithdrawn from the reactor as a polymer product and cause process andreactor failure. Also, chunks entering the downstream handling processof polymer product can disrupt, for example, transfer systems, dryingunits or extruders.

In one preferred embodiment of this present invention, the entry pointfor the recycle stream is preferably below the fluidized bed so as toprovide a uniform flow of the recycle stream to maintain the fluidizedbed in a suspended condition and to ensure uniformity of the recyclestream passing upwardly throughout the fluidized bed. In anotherembodiment of the present invention, the recycle stream can be dividedinto two or more separate streams, one or more of which can beintroduced directly into the fluidized bed provided that the gasvelocity below and through the fluidized bed is sufficient to keep thebed suspended. For example, the recycle stream can be divided into aliquid and a gas stream which can then be separately introduced into thereactor.

In the practice of the improved process of this invention, the recyclestream comprising a mixture of a gas phase and a liquid phase within thereactor below the distributor plate can be formed by separatelyinjecting a liquid and recycle gas under conditions which will produce astream comprising both phases.

The advantages of this invention are not limited to the production ofpolyolefins. Thus, this invention can be practiced in connection withany exothermic reaction carried out in a gas fluidized bed. Theadvantages of a process operating in condensed mode over other processesgenerally increase directly with the nearness of the dew pointtemperature of the recycle steam to the reaction temperature within theinterior of the fluidized bed. For a given dew point, advantages of theprocess may increase directly with the percentage of liquid in therecycle stream returned to the reactor. The invention allows highpercentages of liquid to be used in the process.

A gas fluidized bed reactor which is particularly well suited toproduction of polymers by the process of the present invention is bestillustrated in the accompanying drawing, generally designated in FIG. 1by numeral 10. It should be noted that the reaction system depicted inFIG. 1 is intended to be merely exemplary. The present invention is wellsuited for any conventional fluidized bed reaction systems.

Referring now to FIG. 1, the reactor 10 comprises a reaction zone 12 anda freeboard zone which in this instance is also a velocity reductionzone 14. The height to diameter ratio of the reaction zone 12 can varydepending on the desired production capacity and residence time. Thereaction zone 12 includes a fluidized bed comprising growing polymerparticles, existing formed polymer particles and small amounts ofcatalyst. The fluidized bed in the reaction zone 12 is supported by arecycle stream or fluidizing medium 16 generally made up from feed andrecycle fluids. The recycle stream enters the reactor through adistributor plate 18 in the bottom section of the reactor which aids inthe uniform fluidization and the support of the fluidized bed in thereaction zone 12. In order to maintain the reaction zone fluidized bed12 in a suspended and viable state, the superficial gas velocity (SGV)of the gas flow through the reactor generally exceeds the minimum flowrequired for fluidization which is typically from about 0.2 ft/sec(0.061 m/s) to 0.5 ft/sec (0.153 m/s). Preferably, the SGV must bemaintained at not less than about 0.7 ft/sec (0.214 m/s), and even morepreferably not less than 1.0 ft/sec (0.305 m/s). The SGV shouldpreferably not exceed 5.0 ft/sec (1.5 m/s), especially 3.5 ft/sec (1.07m/s).

Polymer particles in the reaction zone 12 help to prevent the formationof localized "hot spots" and entrap and distribute catalyst particlesthroughout the fluidized bed. In operation, on start up, the reactor 10is charged with a base of polymer particles before the recycle stream 16flow is introduced. These polymer particles are preferably the same asthe new polymer particles to be produced, however, if different, theyare withdrawn with the newly formed first product after initiation ofrecycle and catalyst flows and establishment of reaction. This mixtureis generally segregated from the later essentially new production foralternate disposition. The catalysts used in the improved process ofthis invention are usually sensitive to oxygen, therefore, the catalystis preferably stored in a catalyst reservoir 20 under a blanket of agas, inert to the stored catalyst, such as, but not limited to nitrogenor argon.

Fluidization of the fluidized bed in the reaction zone 12 is achieved bythe high rate at which the recycle stream 16 flows into and through thereactor 10. Typically in operation, the rate of the recycle stream 16 isapproximately ten to fifty times the rate at which the feed isintroduced into the recycle stream 16. This high rate of the recyclestream 16 provides the superficial gas velocity necessary to suspend andmix the fluidized bed in the reaction zone 12 in a fluidized state.

The fluidized bed has a general appearance similar to that of avigorously boiling liquid, with a dense mass of particles in individualmotion caused by percolation and bubbling of gas through the fluidizedbed. As the recycle stream 16 passes through the fluidized bed in thereaction zone 12, there is a pressure drop. This pressure drop is equalto or slightly greater than the weight of the fluidized bed in thereaction zone 12 divided by the cross-sectional area of the reactionzone 12, therefore making the pressure drop dependent on the reactorgeometry.

Again referencing FIG. 1, the make-up feed enters the recycle stream 16at, but not limited to, a point 22. A gas analyzer 24 receives gassamples from the recycle stream line 16 and monitors the composition ofthe recycle stream 16 passing there-through. The gas analyzer 24 is alsoadapted to regulate the composition of the recycle stream line 16 andthe feed to maintain a steady state in the composition of the recyclestream 16 in the reaction zone 12. The gas analyzer 24 usually analyzessamples taken from the recycle stream line 16 at a point between thefreeboard zone 14 and a heat exchanger 26, preferably, between acompressor 28 and the heat exchanger 26.

The recycle stream 16 passes upward through the reaction zone 12adsorbing heat generated by this polymerization process. That portion ofthe recycle stream 16 that does not react in the reaction zone 12 exitsthe reaction zone 12 and passes through the velocity reduction orfreeboard zone 14. As previously described, in this region, the velocityreduction zone 14, a major portion Of entrained polymer drops back intothe fluidized bed reaction zone 12 thereby reducing the carryover ofsolid polymer particles into the recycle stream line 16. The recyclestream 16 once withdrawn from the reactor above the freeboard zone 14 isthen compressed in compressor 28 and passes through the heat exchanger26, where heat generated by the polymerization reaction and gascompression are removed from the recycle stream 16 before returning therecycle stream 16 back to the reaction zone 12 in the reactor 10. Theheat exchanger 26 is conventional in type and can be placed within therecycle stream line 16 in either a vertical or horizontal position. Inan alternative embodiment of this invention, more than one heatexchanging zone or compression zone within the recycle stream line 16may be included.

Referring back to FIG. 1, the recycle stream 16 upon exiting the heatexchanger 26 returns to the bottom of the reactor 10. Preferably, afluid flow deflector 30 is positioned below the gas distributor plate18. The fluid flow deflector 30 prevents polymer from settling out intoa solid mass and maintains entrainment of liquid and polymer particleswithin the recycle stream 16 below the distributor plate 18. Thepreferred type of fluid flow deflector plate is annular disc in shape,for example, the type described in U.S. Pat. No. 4,933,149. Using anannular type disc provides both a central upward and outer peripheralflow. The central upward flow assists in the entrainment of liquiddroplets in the bottom head and the outward peripheral flow assists inminimizing buildup of polymer particles in the bottom head. Thedistributor plate 18 diffuses the recycle stream 16 to avoid the streamentering the reaction zone 12 in a centrally disposed upwardly movingstream or jet that would disrupt fluidization of the fluidized bed inthe reaction zone 12.

The temperature of the fluidized bed is set dependent on the particlesticking point but is basically dependent on three factors: (1) thecatalyst activity and rate of catalyst injection which controls the rateof polymerization and the attendant rate of heat generation, (2) thetemperature, pressure and composition of recycle and makeup streamsintroduced into the reactor and (3) the volume of the recycle streampassing through the fluidized bed. The amount of liquid introduced intothe bed either with the recycle stream or by separate introduction asdescribed previously especially affects the temperature because theliquid vaporizes in the reactor and serves to reduce the temperature ofthe fluidized bed. Usually the rate of catalyst addition is used tocontrol the rate of polymer production.

The temperature of the fluidized bed in the reaction zone 12 in thepreferred embodiment remains constant in a steady state by continuouslyremoving the heat of reaction. A steady state of the reaction zone 12occurs when the amount of heat generated in the process is balanced withthe amount of heat removed. This steady state requires that the totalquantity of material entering the polymerization process is balanced bythe amount of polymer and other material removed. Consequently, thetemperature, the pressure, and the composition at any given point in theprocess is constant with time. There is no significant temperaturegradient within most of the fluidized bed in the reaction zone 12,however, there is a temperature gradient in the bottom of the fluidizedbed in the reaction zone 12 in the region above the gas distributorplate 18. This gradient results from the difference between thetemperature of the recycle stream 16 entering through the distributorplate 18 at the bottom of the reactor 10 and temperature of thefluidized bed in the reaction zone 12.

Efficient operation of the reactor 10 requires good distribution of therecycle stream 16. Should growing or formed polymer and catalystparticles be allowed to settle out of the fluidized bed, fusion of thepolymer can occur. This can result, in an extreme case, in the formationof a solid mass throughout the reactor. A commercial-sized reactorcontains thousands of pounds or kilograms of polymer solids at any giventime. The removal of a solid mass of polymer of this magnitude wouldentail great difficulty, requiring substantial effort and an extendeddowntime. By determining stable operating conditions with the help ofFBD measurement improved polymerization processes can be performed inwhich the fluidization and support of fluidized bed in the reaction zone12 within the reactor 10 are maintained.

In the preferred embodiment, variations in the fluidized bulk density(FBD) for a given grade of polymer and/or catalyst composition are usedto optimize process conditions and plant design. The fluidized bulkdensity is the ratio of the measured pressure drop upward across acentrally fixed portion of the reactor to the height of this fixedportion. It is a mean value which may be greater or less than thelocalized bulk density at any point in the fixed reactor portion. Itshould be understood that under certain conditions known to thoseskilled in the art, a mean value may be measured which is greater orless than the localized bed bulk density.

Applicants have discovered that as the concentration of condensablecomponent is increased in the gaseous stream flowing through the bed, anidentifiable point may be reached beyond which there is danger offailure of the process if the concentration is further increased. Thispoint is characterized by an irreversible decrease in the fluidized bulkdensity with an increase in condensable fluid concentration in the gas.The liquid content of the recycle stream entering the reactor may not bedirectly relevant. The decrease in fluidized bulk density generallyoccurs with no corresponding change in the settled bulk density of thefinal product granules. Thus, the change in fluidization behaviorreflected by the decrease in fluidized bulk density apparently does notinvolve any permanent change in the characteristics of the polymerparticles.

The gas condensable fluid concentrations at which decreases in fluidizedbulk density occur depend upon the type of polymer being produced andother process conditions. They may be identified by monitoring thefluidized bulk density as condensable fluid concentrations in the gasare increased for a given type of polymer and other process conditions.

The fluidized bulk density depends on other variables in addition to thecondensable fluid concentration in the gas, including for example thesuperficial velocity of the gas flowing through the reactor, thefluidized bed height and the settled bulk density of the product as wellas gas and particle densities, temperature and pressure. Thus, in teststo determine changes in fluidized bulk density attributable to changesin gas condensable fluid concentration, significant changes in otherconditions should be avoided. Therefore, it would not be beyond thescope of this invention to monitor these other variables from whichfluidized bulk density can be determined or affects bed instabilities.For the purposes of this application monitoring or maintaining fluidizedbulk density includes monitoring or maintaining those variablesdescribed above that affect fluidized bulk density or are used todetermine fluidized bulk density.

While some modest drop in fluidized bulk density may be accommodatedwithout the loss of control, further changes in gas composition or othervariables which also increase the dew point temperature may beaccompanied by an irreversible decrease in the fluidized bulk density,development of "hot spots" in the reactor bed, formation of fusedagglomerates and eventual shutdown of the reactor.

Other practical consequences directly related to the reduction of thefluidized bulk density include a reduced polymer capacity of afixed-volume reactor discharge system and reduced polymer/catalystreactor residence time at constant polymer production rate. The lattermay, for a given catalyst, reduce the catalyst productivity and increasethe level of catalyst residues in the product polymer. In a preferredembodiment it is desirable to minimize the condensable fluidconcentration in the gas for a given target reactor production rate andassociated cooling requirement.

Using such fluidized bulk density variations, stable operatingconditions can be defined. Once a suitable composition has beenidentified, the composition may be used to achieve much higher coolingcapacities for the recycle stream (without encountering bedinstabilities) by cooling that composition to a greater degree.Condensable, non-polymerizable materials may be added in appropriateamounts for a particular grade to achieve high reactor productivitywhilst preserving good conditions in the fluidized bed by staying withinthe so determined stable operating zone. High reactor productivity canbe achieved in a process or, in terms of plant design, a large capacityplant can be designed with a relatively small reactor diameter orexisting reactors can be modified to provide increased capacity withoutchanging the reactor size.

At higher reactor productivities it has been found that, staying withinthe boundaries defined by the acceptable fluidized bulk density changes,levels of condensed liquid well over 15%, 20%, 22%, 25% or even 30% canbe accommodated whilst avoiding significant levels of chunking orsheeting resulting from fluidized bed disruption. The levels ofcondensed liquid based on the total weight of the recycle stream orfluidizing medium is in the range of between 15-50 weight percent,preferably greater than 20 to 50 weight percent and even more preferably22-50 weight percent, and most preferably 25-50 weight percent.

Preferably the fluidized bulk density is observed by using a pressuredifference measurement from a part of the fluidized bed not prone todisturbances over the distributor plate. Whereas conventionallyfluidized bulk density variations in the lower part of the bed can betaken to be indicative of bed disruption over the distributor plate,with the upper fluidized bulk density measured remote from thedistributor plate being used as a stable reference, it has nowsurprisingly been found that the changes in the upper fluidized bulkdensity correlate to change in the composition of the stream and can beused to find and define stable operating zones.

Advantageously the recycle stream is cooled and passes at a velocitythrough the reactor such that the cooling capacity is sufficient for areactor productivity expressed in pounds (lbs) of polymer per hr/ft² ofreactor cross-sectional area exceeding 500 lb/hr-ft² (2441 kg/hr-m²),especially 600 lb/hr-ft² (2929 kg/hr-m²) involving an enthalpy change ofthe recycle stream from the reactor inlet conditions to the reactoroutlet conditions of at least 40 Btu/lb, preferably 50 Btu/lb.Preferably, the liquid and gaseous component of the stream are added ina mixture below a reactor distributor plate. This reactor productivityis equal to the Space time yield multiplied by the height of thefluidized bed.

In the preferred embodiment of the present invention, the liquidintroduced into the reactor 10 is vaporized in order to achieve theincreased reactor cooling capacity benefits of this polymerizationprocess. High levels of liquid in the bed may promote the formation ofagglomerates which cannot be broken up by mechanical forces present inthe bed, thus leading potentially to defluidization, bed collapse andreactor shutdown. In addition, the presence of liquids can influencelocal bed temperatures and affect the capability of the process toproduce polymer having consistent properties, since this requires anessentially constant temperature throughout the bed. For these reasons,the amount of liquid introduced into the fluidized bed under a given setof conditions should not materially exceed the amount that will vaporizein the lower region of the fluidized bed, where mechanical forcesassociated with entry of the recycle stream through the distributorplate are sufficient to break up agglomerates formed by liquid-particleinteraction.

It has been discovered in this present invention that, for givencomposition and physical characteristics of the product particles in thefluidized bed and otherwise given or related reactor and recycleconditions, by defining boundary conditions related to the compositionof the gas flowing through the bed, a viable fluidized bed can bemaintained at high cooling levels.

While not wishing to bound by any theory, applicants suggest that theobserved decrease in fluidized bulk density may reflect an expansion ofthe dense particulate phase and change in bubble behavior within thefluidized bed.

Referring back to FIG. 1, a catalyst activator, if required depending onthe catalyst utilized, is generally added downstream from the heatexchanger 26. The catalyst activator may be introduced from a dispenser32 into the recycle stream 16. However, the improved process of thispresent invention is not limited to the location of the insertion of thecatalyst activator or any other required components such as catalystpromoters.

The catalyst from the catalyst reservoir can be injected eitherintermittently or continuously into the fluidized bed reaction zone 12at a preferred rate at a point 34 which is above the gas distributorplate 18. In the preferred embodiment as described above, the catalystis injected at a point where mixing with polymer particles within thefluidized bed 12 is best accomplished. Because some catalysts are veryactive, the preferred injection into the reactor 10 should be above thegas distributor plate 18, not below. Injection of catalyst in the areabelow the gas distributor plate 18 may result in the polymerization ofproduct in this area, which would result eventually in the plugging ofthe gas distributor plate 18. Also, introducing the catalyst above thegas distributor plate 18 aids in the uniform distribution of catalystthroughout the fluidized bed 12 and, therefore, helps to preclude theformation of "hot spots" resulting from high local catalystconcentrations. Injection is preferably into the lower portion of thefluidized bed in the reaction zone 12 to provide uniform distributionand to minimize catalyst carryover into the recycle line wherepolymerization may lead to eventual plugging of the recycle line andheat exchanger.

A variety of techniques for catalyst injection may be utilized in theimproved process of this present invention, for example the techniquedescribed in U.S. Pat. No. 3,779,712, the disclosure of which isincorporated herein by reference. An inert gas such as nitrogen or aninert liquid that readily volatilizes under reactor conditions ispreferably used to carry the catalyst into the fluidized bed reactionzone 12. The catalyst injection rate and monomer concentration in therecycle stream 16 determines the rate of polymer production in thefluidized bed reaction zone 12. It is possible to control the productionrate of the polymer produced by simply adjusting catalyst injectionrate.

In the preferred operating mode of the reactor 10 utilizing the improvedprocess of this present invention, the height of the fluidized bed inreaction zone 12 is maintained by the withdrawal of a portion of thepolymer product at a rate consistent with the formation of the polymerproduct. Instrumentation for detecting any temperature or pressurechanges throughout the reactor 10 and recycle stream 16 are useful tomonitor changes in the condition of the fluidized bed in the reactionzone 12. Also, this instrumentation allows for either the manual orautomatic adjustment of rate of catalyst injection or the temperature ofthe recycle stream.

In operation of the reactor 10, the product is removed from the reactorthrough a discharge system 36. The discharge of polymer product ispreferably followed by separating fluids from the polymer product. Thesefluids may be returned to the recycle stream line 16 as a gas at point38 and/or as a condensed liquid at point 40. The polymer product isrouted to downstream processing at point 42. The discharge of polymerproduct is not limited to the method shown in FIG. 1, which illustratesjust one particular discharge method. Other discharge systems can beemployed, for example, those disclosed and claimed in U.S. Pat. Nos.4,543,399, and 4,588,790 to Jenkins, et al.

In accordance with the present invention, a process is provided forincreasing the reactor productivity of polymer production in a fluidizedbed reactor employing an exothermic polymerization reaction by coolingthe recycle stream to below its dew point and returning the resultantrecycle stream to the reactor. The recycle stream containing greaterthan 20 weight percent liquid can be recycled to the reactor to maintainthe fluidized bed at a desired temperature.

In the processes of the invention, the cooling capacity of the recyclestream or fluidizing medium may be significantly increased by both thevaporization of the condensed liquids entrained in the recycle streamand as a result of the greater temperature differential between theentering recycle stream and the fluidized bed temperature. In thepreferred embodiment the polymers, homopolymers or copolymers, producedare selected from a film grade material having a MI from 0.01 to 5.0,preferably 0.5 to 5.0 and a density of 0.900 to 0.930; or a moldinggrade material having a MI of from 0.10 to 150.0, preferably 4.0 to150.0 and a density of from 0.920 to 0.939; or a high density materialhaving a MI of from 0.01 to 70.0, preferably 2.0 to 70.0 and a densityof from 0.940 to 0.970; all density units being in a g/cm³ and the meltindex being in g/10 min determined according to ASTM-1238 condition E.

Depending on the target material different recycle conditions may beadopted providing reactor productivity levels not previously envisaged.

Firstly, there may be produced for example a film grade material inwhich the recycle stream has a butene/ethylene mole ratio of from 0.001to 0.60, preferably 0.30 to 0.50 or a 4-methyl-pentene-1/ethylene moleratio of from 0.001 to 0.50 preferably 0.08 to 0.33 or a hexene/ethylenemole ratio of from 0.001 to 0.30, preferably 0.05 to 0.20; or anoctene-1/ethylene mole ratio of from 0.001 to 0.10, preferably 0.02 to0.07; a hydrogen/ethylene mole ratio of from 0.00 to 0.4, preferably 0.1to 0.3; and an isopentane level of from 3 to 20 mol % or an isohexanelevel of from 1.5 to 10 mol % and in which the cooling capacity of therecycle stream is at least 40 Btu/lb, preferably at least 50 but/lb orthe weight percent condensed is at least 15, preferably greater than 20.

Secondly, the process may be used to yield a molding grade material inwhich the recycle stream has a butene-1/ethylene mole ratio of from0.001 to 0.60, preferably 0.10 to 0.50 or a 4-methyl-pentene-1/ethylenemole ratio of from 0.001 to 0.50, preferably, 0.08 to 0.20 or ahexene/ethylene mole ratio of from 0.001 to 0.30, preferably 0.05 to0.12 or an octene-1/ethylene mole ratio of from 0.001 to 0.10,preferably 0.02 to 0.04; a hydrogen/ethylene mole ratio of from 0.00 to1.6, preferably 0.3 to 1.4; and an isopentane level of from 3 to 30 mol% or an isohexane level of from 1.5 to 15 mol % and in which the coolingcapacity of the recycle stream is at least 40 Btu/lb, preferably atleast 50 Btu/lb or the weight percent condensed is at least 15,preferably greater than 20.

Also, high density grades may be made by a process in which the recyclestream has a butene-ethylene mole ratio of 0.001 to 0.30, preferably0.001 to 0.15 or a 4-methyl-pentene-1/ethylene mole ratio of from 0.001to 0.25, preferably 0.001 to 0.12 or a hexene/ethylene mole ratio of0.001 to 0.15, preferably 0.001 to 0.07 or an octene-1/ethylene moleratio of from 0.001 to 0.05, preferably 0.001 to 0.02; a hydrogen toethylene mole ratio of 0.00 to 1.5, preferably 0.3 to 1.0; and anisopentane level of from 10 to 40 mol % or an isohexane level of from 5to 20 mol % and in which the cooling capacity of the recycle stream isat least 60 Btu/lb, preferably greater than 73 Btu/lb, and mostpreferably greater than at least about 75 Btu/lb or the weight percentcondensed is at least 12, preferably greater than 20.

EXAMPLES

In order to provide a better understanding of the present inventionincluding representative advantages and limitations thereof, thefollowing referential examples are offered as related to actual testsperformed in the practice of this invention.

Example 1

A fluidized gas phase reactor was operated to produce a copolymercontaining ethylene and butene. The catalyst used is a complex oftetrahydrofuran, magnesium chloride and titanium chloride reduced withdiethyl aluminum chloride (diethyl aluminum chloride-to-tetrahydrofuranmolar ratio of 0.50) and tri-n-hexyl aluminum (tri-n-hexylaluminum-to-tetrahydrofuran molar ratio of 0.30) impregnated on triethylaluminum treated silicon dioxide. The activator is triethyl aluminum(TEAL).

The data in Table 1 and illustrated in FIG. 2 shows the reactorparameters as the isopentane level is gradually increased to achieve theadded cooling necessary to obtain higher reactor productivity. Thisexample shows that excessive amounts of isopentane leads to changes inthe fluidized bed and ultimately to its disruption in the formation ofhot spots and agglomerates necessitating reactor shut-down. As theconcentration of isopentane increases the fluidized bulk densitydecreases indicating a change in the bed fluidization which alsoresulted in an increase in the bed height. The catalyst rate wasdecreased to reduce the bed level. In addition, the isopentaneconcentration was reduced in an attempt to reverse the change in thefluidized bed. However, at this point, although the bed height returnedto normal, the disruption accompanied by hot spots and agglomerations inthe bed was irreversible and the reactor was shut-down.

                                      TABLE 1                                     __________________________________________________________________________                           Time (Hours)                                                                  1    7    10   13   15   17   18                       __________________________________________________________________________    Resin Melt Index (dg/10 min)                                                                         1.01 1.04 1.03 1.12 1.09 1.11 1.11                     Resin Density (g/cc)   0.9176                                                                             0.9183                                                                             0.9190                                                                             0.9190                                                                             0.9183                                                                             0.9193                                                                             0.9193                   Recycle Stream Compositions:                                                  Ethylene               47.4 46.0 44.7 44.1 44.0 45.9 46.3                     Butene-1               19.0 18.1 17.3 17.0 16.9 18.5 19.5                     Hexene-1                                                                      Hydrogen               9.5  9.4  9.3  9.3  8.9  8.7  8.9                      Isopentane             8.0  10.8 13.7 15.1 15.4 14.3 13.2                     C.sub.6 Saturated Hydrocarbons                                                Nitrogen               14.3 13.9 13.3 12.8 13.2 11.2 10.7                     Ethane                 1.8  1.8  1.7  1.7  1.6  1.4  1.4                      Methane                                                                       C.sub.8 Saturated Hydrocarbons                                                Recycle Gas Dew Point (°F.)                                                                   142.9                                                                              153.5                                                                              163.8                                                                              168.3                                                                              170.1                                                                              168.8                                                                              165.0                    Recycle Gas Dew Point (°C.)                                                                   61.6 67.5 73.2 75.7 76.7 76.0 73.9                     Reactor Inlet Temperature (°F.)                                                               126.2                                                                              135.6                                                                              143.5                                                                              144.0                                                                              149.0                                                                              150.2                                                                              146.3                    Reactor Inlet Temperature (°C.)                                                               52.3 57.6 61.9 62.2 65.0 65.7 63.5                     Liquid in Recycle gas (wt %)                                                                         11.4 12.1 14.3 17.4 14.5 11.6 12.3                     Reactor Temperature (°F.)                                                                     182.4                                                                              182.1                                                                              182.7                                                                              182.8                                                                              183.1                                                                              184.8                                                                              185.2                    Reactor Temperature (°C.)                                                                     83.6 83.4 83.7 83.8 83.9 84.9 85.1                     Reactor Pressure (psig)                                                                              311.9                                                                              311.5                                                                              314.2                                                                              313.4                                                                              314.7                                                                              313.5                                                                              312.6                    Reactor Pressure (kPag)                                                                              2150.5                                                                             2147.7                                                                             2166.3                                                                             2160.8                                                                             2169.8                                                                             2161.5                                                                             2155.3                   Reactor Superficial Gas Velocity (Ft/sec)                                                            2.29 2.30 2.16 2.10 1.92 2.00 2.11                     Reactor Superficial Gas Velocity (m/sec)                                                             0.70 0.70 0.66 0.64 0.59 0.61 0.64                     Reactor Bed Height (ft)                                                                              43.4 43.3 43.5 49.3 51.3 45.8 45.4                     Reactor Bed Height (m) 13.2 13.2 13.3 15.0 15.6 14.0 13.8                     Resin Settled Bulk Density (lb/ft.sup.3)                                                             30.1 30.2 30.2 30.2 30.0 29.9 29.9                     Resin Settled Bulk Density (kg/m.sup.3)                                                              482.2                                                                              483.8                                                                              483.8                                                                              483.8                                                                              480.6                                                                              479.0                                                                              479.0                    Reactor Bed Fluidized Bulk Density (lb/ft.sup.3)                                                     18.9 19.6 18.1 17.8 17.2 16.4 15.8                     Reactor Bed Fluidized Bulk Density (kg/m.sup.3)                                                      302.8                                                                              314.0                                                                              290.0                                                                              285.2                                                                              275.5                                                                              262.7                                                                              253.1                    Ratio of Fluidized Bulk Density to Settled Bulk                                                      0.63 0.65 0.60 0.59 0.57 0.55 0.53                     Density                                                                       Space Time Yield (lb/hr-ft.sup.3)                                                                    9.6  9.5  9.3  8.5  6.6  7.1  7.3                      Space Time Yield (kg/hr-m.sup.3)                                                                     153.0                                                                              151.8                                                                              149.3                                                                              136.0                                                                              106.0                                                                              113.8                                                                              117.2                    Production Rate (klb/hr)                                                                             68.5 67.8 67.0 69.2 56.1 53.8 54.9                     Production Rate (Tons/hr)                                                                            31.1 30.7 30.4 31.4 25.4 24.4 24.9                     Reactor Productivity (lb/hr-ft.sup.2)                                                                415  411  406  419  340  326  332                      Reactor Productivity (kg/hr-m.sup.2)                                                                 2026 2006 1982 2045 1660 1591 1621                     Recycle Stream Enthalpy Change (Btu/lb)                                                              42   40   40   42   37   34   33                       Recycle Stream Enthalpy Change (cal/g)                                                               23   22   22   23   21   19   18                       __________________________________________________________________________

Furthermore, in a second run, Table 2 and FIG. 3 shows that as theconcentration of isopentane was gradually increased the fluidized bulkdensity decreased as expected from Table 1. However, this time thefluidized bulk density gradually increased as a result of reducing theconcentration of isopentane. Thus, in this instance, the change influidization in the bed was recoverable and reversible.

                                      TABLE 2                                     __________________________________________________________________________                      Time (Hours)                                                                  1    3    5    7    9    11   14   16   18                  __________________________________________________________________________    Resin Melt Index (dg/10 min)                                                                    0.92 0.99 1.08 1.02 1.05 1.09 1.11 1.05 0.98                Resin Density (g/cc)                                                                            0.9187                                                                             0.9184                                                                             0.9183                                                                             0.9181                                                                             0.9178                                                                             0.9177                                                                             0.9186                                                                             0.9184                                                                             0.9183              Recycle Stream Compositions:                                                  Ethylene          52.6 53.2 52.6 52.0 52.1 51.6 52.9 52.8 52.8                Butene-1          20.0 19.8 19.7 20.4 19.7 19.8 19.1 20.1 20.1                Hexene-1                                                                      Hydrogen          9.7  10.2 10.3 9.9  9.9  9.9  10.4 10.0 9.6                 Isopentane        9.9  9.5  10.7 11.2 12.2 12.8 11.5 10.4 9.6                 C.sub.6 Saturated Hydrocarbons                                                Nitrogen          8.7  8.0  7.3  6.7  6.3  6.0  6.5  7.3  8.1                 Ethane            1.2  1.2  1.1  1.1  1.1  1.1  1.2  1.2  1.3                 Methane                                                                       C.sub.8 Saturated Hydrocarbons                                                Recycle Gas Dew Point (°F.)                                                              154.1                                                                              152.5                                                                              156.9                                                                              160.0                                                                              161.9                                                                              165.0                                                                              159.4                                                                              155.9                                                                              153.3               Recycle Gas Dew Point (°C.)                                                              67.8 66.9 69.4 71.1 72.2 73.9 70.8 68.8 67.4                Reactor Inlet Temperature (°F.)                                                          124.2                                                                              118.3                                                                              119.7                                                                              125.3                                                                              127.3                                                                              133.2                                                                              128.0                                                                              126.2                                                                              123.0               Reactor Inlet Temperature (°C.)                                                          51.2 47.9 48.7 51.8 52.9 56.2 53.3 52.3 50.6                Liquid in Recycle gas (wt %)                                                                    22.2 24.9 27.4 26.4 27.0 24.3 23.2 22.1 22.2                Reactor Temperature (°F.)                                                                184.6                                                                              185.2                                                                              184.1                                                                              183.4                                                                              183.5                                                                              183.3                                                                              182.8                                                                              181.9                                                                              181.8               Reactor Temperature (°C.)                                                                84.8 85.1 84.5 84.1 84.2 84.0 83.8 83.3 83.2                Reactor Pressure (psig)                                                                         314.7                                                                              315.2                                                                              315.2                                                                              315.1                                                                              315.3                                                                              314.8                                                                              312.9                                                                              312.9                                                                              313.4               Reactor Pressure (kPag)                                                                         2170.0                                                                             2173.3                                                                             2173.3                                                                             2172.5                                                                             2174.2                                                                             2170.7                                                                             2157.6                                                                             2157.7                                                                             2160.6              Reactor Superficial Gas Velocity                                                                1.73 1.74 1.75 1.76 1.77 1.76 1.75 1.74 1.74                (Ft/sec)                                                                      Reactor Superficial Gas Velocity                                                                0.53 0.53 0.53 0.54 0.54 0.54 0.53 0.53 0.53                (m/sec)                                                                       Reactor Bed Height (ft)                                                                         44.7 45.0 44.6 44.9 46.0 47.0 45.5 45.6 45.2                Reactor Bed Height (m)                                                                          13.6 13.7 13.6 13.7 14.0 14.3 13.9 13.9 13.8                Resin Settled Bulk Density (lb/ft.sup.3)                                                        29.9 29.9 29.7 28.8 29.0 29.1 29.3 29.4 29.4                Resin Settled Bulk Density (kg/m.sup.3)                                                         479.0                                                                              479.0                                                                              475.8                                                                              461.4                                                                              464.6                                                                              465.4                                                                              468.6                                                                              471.3                                                                              471.8               Reactor Bed Fluidized Bulk Density                                                              20.2 20.7 19.6 19.3 18.2 17.1 18.5 19.2 20.0                (lb/ft.sup.3)                                                                 Reactor Bed Fluidized Bulk Density                                                              323.9                                                                              330.9                                                                              314.4                                                                              309.9                                                                              291.1                                                                              274.3                                                                              296.2                                                                              308.1                                                                              321.1               (kg/m.sup.3)                                                                  Ratio of Fluidized Bulk Density to                                                              .68  .69  .66  .67  .63  .59  .63  .65  .68                 Settled Bulk Density                                                          Space Time Yield (lb/hr-ft.sup.3)                                                               9.7  10.3 11.1 11.1 11.1 9.9  9.3  9.1  9.2                 Space Time Yield (kg/hr-m.sup.3)                                                                154.9                                                                              165.1                                                                              178.1                                                                              178.0                                                                              177.0                                                                              158.4                                                                              149.1                                                                              144.9                                                                              147.3               Production Rate (klb/hr)                                                                        71.3 76.6 82.2 82.3 84.0 76.8 69.9 68.0 68.5                Production Rate (Tons/hr)                                                                       32.3 34.7 37.3 37.3 38.1 34.8 31.7 30.8 31.1                Reactor Productivity (lb/hr-ft.sup.2)                                                           432  464  498  498  509  465  423  412  415                 Reactor Productivity (kg/hr-m.sup.2)                                                            2109 2265 2431 2431 2485 2270 2065 2011 2026                Recycle Stream Enthalpy Change                                                                  54   59   61   60   61   55   52   51   52                  (Btu/lb)                                                                      Recycle Stream Enthalpy Change                                                                  30   33   34   33   34   31   29   28   29                  (cal/g)                                                                       __________________________________________________________________________

Therefore, FIG. 4, a representation of the results of both FIGS. 1 and2, clearly illustrates a point at which changes in bed fluidization arenot reversible because of the excessive use of a condensable fluid. Thispoint is defined to be where the ratio of the reactor bed fluidized bulkdensity to the settled bulk density is less than 0.59. Example 1 clearlydemonstrates, in contrast to the disclosure in Jenkins, et al., thatthere is a limit for condensable materials useful to optimize the spacetime yield or reactor productivity of a reactor operating in condensedmode.

Example 2

The following examples were carried out in essentially the same way asExample 1 utilizing the same type of catalyst and activator to producehomopolymers and ethylene/butene copolymers of various density and meltindex ranges.

                                      TABLE 3                                     __________________________________________________________________________                           Run                                                                           1    2    3    4    5                                  __________________________________________________________________________    Resin Melt Index (dg/10 min)                                                                         0.86 6.74 7.89 22.22                                                                              1.91                               Resin Density (g/cc)   0.9177                                                                             0.9532                                                                             0.9664                                                                             0.9240                                                                             0.9186                             Recycle Stream Compositions:                                                  Ethylene               53.1 40.5 49.7 34.1 44.0                               Butene-1               20.2           14.9 18.2                               Hexene-1                    0.6                                               Hydrogen               8.9  17.7 26.5 25.0 11.9                               Isopentane             9.7  3.7  0.7  14.1 9.6                                C.sub.6 Saturated Hydrocarbons                                                                            7.0  10.2                                         Nitrogen               8.7  19.2 8.8  9.4  14.9                               Ethane                 1.7  9.4  4.0  2.5  3.3                                Methane                     1.1  0.3                                          C.sub.8 Saturated Hydrocarbons                                                                            0.4  0.5                                          Recycle Gas Dew Point (°F.)                                                                   154.0                                                                              172.6                                                                              181.6                                                                              162.1                                                                              148.5                              Recycle Gas Dew Point (°C.)                                                                   67.8 78.1 83.1 72.3 64.7                               Reactor Inlet Temperature (°F.)                                                               115.2                                                                              107.8                                                                              117.7                                                                              135.0                                                                              114.2                              Reactor Inlet Temperature (°C.)                                                               46.2 42.1 47.6 57.2 45.7                               Liquid in Recycle gas (wt %)                                                                         28.6 25.4 27.6 21.8 24.4                               Reactor Temperature (°F.)                                                                     183.3                                                                              208.4                                                                              209.3                                                                              178.0                                                                              183.7                              Reactor Temperature (°C.)                                                                     84.1 98.0 98.5 81.1 84.3                               Reactor Pressure (psig)                                                                              315.7                                                                              300.2                                                                              299.8                                                                              314.7                                                                              314.3                              Reactor Pressure (kPag)                                                                              2176.7                                                                             2069.7                                                                             2066.8                                                                             2169.8                                                                             2167.2                             Reactor Superficial Gas Velocity (Ft/sec)                                                            1.69 2.76 2.36 1.74 1.73                               Reactor Superficial Gas Velocity (m/sec)                                                             0.52 0.84 0.72 0.53 0.53                               Reactor Bed Height (ft)                                                                              47.2 43.0 42.0 44.3 45.6                               Reactor Bed Height (m) 14.4 13.1 12.8 13.5 13.9                               Resin Settled Bulk Density (lb/ft.sup.3)                                                             28.3 23.2 29.0 24.5 29.3                               Resin Settled Bulk Density (kg/m.sup.3))                                                             453.4                                                                              371.0                                                                              464.0                                                                              392.5                                                                              468.6                              Reactor Bed Fluidized Bulk Density (lb/ft.sup.3)                                                     19.6 16.7 21.7 15.7 19.1                               Reactor Bed Fluidized Bulk Density (kg/m.sup.3)                                                      314.0                                                                              267.9                                                                              347.4                                                                              251.5                                                                              305.7                              Ratio of Fluidized Bulk Density to Settled                                                           0.69 0.72 0.75 0.64 0.65                               Bulk Density                                                                  Space Time Yield (lb/hr-ft.sup.3)                                                                    10.8 14.3 13.0 7.7  9.8                                Space Time Yield (kg/hr-m.sup.3)                                                                     172.8                                                                              228.8                                                                              208.0                                                                              123.2                                                                              157.2                              Production Rate (klb/hr)                                                                             83.7 101.2                                                                              90.2 56.6 73.7                               Production Rate (Tons/hr)                                                                            38.0 45.9 40.9 25.7 33.4                               Reactor Productivity (lb/hr-ft.sup.2)                                                                507  613  546  343  446                                Reactor Productivity (kg/hr-m.sup.2)                                                                 2475 2992 2665 1674 2177                               Recycle Stream Enthalpy Change (Btu/lb)                                                              65   67   75   49   60                                 Recycle Stream Enthalpy Change (cal/g)                                                               36   37   42   27   33                                 __________________________________________________________________________

These runs demonstrate the advantages of achieving higher reactorproductivity at levels of condensed liquid exceeding 20 weight percentwhile maintaining the ratio of fluidized bulk density to settled bulkdensity of at least 0.59.

Because of the downstream handling processes, for example, productdischarge systems, extruders and the like, certain reactor conditionshad to be manipulated in order not to exceed the overall plant capacity.Therefore, the full advantages of this invention cannot be fullyappreciated by the Examples shown in Table 3.

For instance, in run 1 of Table 3, the superficial gas velocity was keptlow at around 1.69 ft/sec and therefore, the space-time-yield reflectedis much less than would otherwise be the case. If the velocity wasmaintained at around 2.4 ft/sec the estimated space-time-yield would bein the excess of 15.3 lb/hr-ft³ would be achievable. Runs 2 and 3 ofTable 3 show the effect of operating a reactor at a high superficial gasvelocity and a weight percent condensed well above 20%. Thespace-time-yields achieved were around 14.3 and 13.0 lb/hr-ft³demonstrating a significant increase in production rate. Such high STYor production rates are not taught or suggested by Jenkins, et al.Similar to run 1, run 4 of Table 3 shows a superficial gas velocity of1.74 ft/sec at 21.8 weight percent condensed liquid. If the velocity inrun 4 is increased to 3.0 ft/sec the achievable STY would increase from7.7 to 13.3 lb/hr-ft³. If the velocity in run 5 is increased to 3.0ft/sec the achievable space-time-yield would increase from 9.8 to 17.0lb/hr-ft³. For all runs 1-5 the ratio of the fluidized bulk density tosettled bulk density was maintained above at least 0.59.

Example 3

The data shown for the cases in paper Example 3, Table 4, were preparedby extrapolating information from actual operations by usingthermodynamic equations well known in the art to project targetconditions. This data in Table 4 illustrates the advantages of thisinvention if limitations of auxiliary reactor equipment is removed.

                                      TABLE 4                                     __________________________________________________________________________                           RUN 1               RUN 2                              Case                   1    2    3    4    1    2    3    4                   __________________________________________________________________________    Resin Melt Index (dg/10 min)                                                                         0.86                6.74                               Resin Density (g/cc)   0.9177              0.9532                             Recycle Stream Compositions:                                                  Ethylene               53.1 53.1 53.1 53.1 40.5 40.5 40.5 40.5                Butene-1               20.2 20.2 20.2 20.2                                    Hexene-1                                   0.6  0.6  0.6  0.6                 Hydrogen               8.9  8.9  8.9  8.9  17.7 17.7 17.7 17.7                Isopentane             9.7  9.7  9.7  13.0 3.7  3.7  3.7  3.7                 C.sub.6 Saturated Hydrocarbons             7.0  7.0  10.0 10.0                Nitrogen               8.7  8.7  8.7  5.9  19.2 19.2 17.2 17.2                Ethane                 1.7  1.7  1.7  1.2  9.4  9.4  8.5  8.5                 Methane                                    1.1  1.1  1.0  1.0                 C.sub.8 Saturated Hydrocarbons             0.4  0.4  0.4  0.4                 Recycle Gas Dew Point (°F.)                                                                   154.0                                                                              154.0                                                                              154.0                                                                              167.9                                                                              172.6                                                                              172.6                                                                              188.3                                                                              188.3               Recycle Gas Dew Point (°C.)                                                                   67.8 67.8 67.8 75.5 78.1 78.1 86.8 86.8                Reactor Inlet Temperature (°F.)                                                               115.2                                                                              115.2                                                                              105.0                                                                              105.0                                                                              107.8                                                                              100.0                                                                              100.0                                                                              85.0                Reactor Inlet Temperature (°C.)                                                               46.2 46.2 40.6 40.6 42.1 37.8 37.8 29.4                Liquid in Recycle gas (wt %)                                                                         28.6 28.6 34.4 44.2 25.4 27.1 35.9 38.6                Reactor Temperature (°F.)                                                                     183.3                                                                              183.3                                                                              183.3                                                                              183.3                                                                              208.4                                                                              208.4                                                                              208.4                                                                              208.4               Reactor Temperature (°C.)                                                                     84.1 84.1 84.1 84.1 98.0 98.0 98.0 98.0                Reactor Pressure (psig)                                                                              315.7                                                                              315.7                                                                              315.7                                                                              315.7                                                                              300.2                                                                              300.2                                                                              300.2                                                                              300.2               Reactor Pressure (kPag)                                                                              2176.7                                                                             2176.7                                                                             2176.7                                                                             2176.7                                                                             2069.7                                                                             2069.7                                                                             2069.7                                                                             2069.7              Reactor Superficial Gas Velocity (Ft/sec)                                                            1.69 2.40 2.40 2.40 2.76 2.76 2.76 2.76                Reactor Superficial Gas Velocity (m/sec)                                                             0.52 0.73 0.73 0.73 0.84 0.84 0.84 0.84                Reactor Bed Height (ft)                                                                              47.2 47.2 47.2 47.2 43.0 43.0 43.0 43.0                Reactor Bed Height (m) 14.4 14.4 14.4 14.4 13.1 13.1 13.1 13.1                Space Time Yield (lb/hr-ft.sup.3)                                                                    10.8 15.3 18.1 23.3 14.3 15.6 17.8 19.8                Space Time Yield (kg/hr-m.sup.3)                                                                     172.8                                                                              245.4                                                                              290.3                                                                              372.2                                                                              228.8                                                                              249.9                                                                              284.4                                                                              317.6               Production Rate (klb/hr)                                                                             83.7 118.9                                                                              140.6                                                                              180.3                                                                              101.2                                                                              110.5                                                                              125.8                                                                              140.5               Production Rate (Tons/hr)                                                                            38.0 53.9 63.8 81.7 45.9 50.1 57.0 63.7                Reactor Productivity (lb/hr-ft.sup.2)                                                                507  720  851  1092 613  669  762  851                 Reactor Productivity (kg/hr-m.sup.2)                                                                 2475 3515 4154 5331 2992 3266 3720 4154                Recycle Stream Enthalpy Change (Btu/lb)                                                              67   67   77   95   69   76   81   90                  Recycle Stream Enthalpy Change (cal/G)                                                               37   37   43   53   38   42   45   50                  __________________________________________________________________________

In run 1, the superficial gas velocity is increasing from 1.69 ft/sec to2.40 ft/sec which results in a higher STY of 15.3 lb/hr-ft³ as comparedto the initial 10.8 lb/hr-ft³. In a further step, the recycle inletstream is cooled to 40.6° C. from 46.2° C. This cooling increases therecycle condensed level to 34.4 wt. % and allows additional improvementin STY to 18.1. In the last step, the gas composition is changed byincreasing the concentration of the condensable inert, isopentane,thereby improving the cooling capability. Through this means, therecycle condensed level further increases to 44.2 wt. % and the STYreaches 23.3. Overall, the incremental steps provide a 116% increase inproduction capacity from the reactor system.

In run 2, the recycle inlet temperature is cooled to 37.8° C. from 42.1°C. This cooling increases the recycle condensed from 25.4 wt. % to 27.1wt. % and an increase in STY from 14.3 to 15.6 lb/hr-ft³. In a furtherstep, the concentration of C6 hydrocarbons is increased from 7 mol % to10 mol %. This improvement in cooling capability allows an increase inSTY to 17.8 lb/hr-ft². As a final step to demonstrate the value of thisimprovement, the recycle inlet temperature is again decreased to 29.4°C. This additional cooling allows an STY of 19.8 lb/hr-ft³ as thecondensed level of the recycle stream reaches 38.6 wt. %. Overall, theincremental steps provide a 39% increase in production capacity from thereactor system.

While the present invention has been described and illustrated byreference to particular embodiments thereof, it will be appreciated bythose of ordinary skill in the art that the invention lends itself tovariations not necessarily illustrated herein. For example, it is notbeyond the scope of this invention to utilize a catalyst of increasedactivity to increase the rate of production or reduce the temperature ofa recycle stream by employing refrigerator units. For this reason, then,references should be made solely to the appended claims for purposes ofdetermining the true scope of the present invention.

We claim:
 1. A process for polymerizing alpha-olefin(s) in a gas phasereactor having a fluidized bed and a fluidizing medium wherein thefluidizing medium serves to control the cooling capacity of saidreactor, the improvement comprising employing a level of liquid in thefluidizing medium entering the reactor which is in the range of from17.4 to 50 weight percent based on the total weight of the fluidizingmedium and maintaining the ratio of fluidized bulk density to settledbulk density above 0.59.
 2. The process in accordance with claim 1wherein the level of liquid is within the range of from 22 to 50 weightpercent based on the total weight of the fluidizing medium.
 3. Theprocess in accordance with claim 1 wherein the level of liquid is withinthe range of from 25 to 50 weight percent liquid based on the totalweight of the fluidizing medium
 4. The process in accordance with claim1 wherein the level of liquid in the fluidizing medium based on thetotal weight of the fluidizing medium is in the range of from 30 to 50weight percent.
 5. The process in accordance with claim 1 whereinpolymer product is withdrawn at a rate greater than about 500 lb/hr-ft².6. The process in accordance with claim 1 wherein said fluidizing mediumcomprises:i) butene-1 and ethylene at a molar ratio of from about 0.001to about 0.60 or 4-methyl-pentene-1 and ethylene at a molar ratio offrom about 0.001 to about 0.50 or hexene-1 and ethylene at a molar ratioof from about 0.001 to about 0.30 or octene-1 and ethylene at a molarratio of from about 0.001 to about 0.10; ii) a condensable inert fluidcomprising from about 1.5 to about 20 mole percent of the fluidizingmedium.
 7. The process in accordance with claim 6 wherein hydrogen ispresent at a molar ratio with respect to ethylene of from about 0.00 toabout 0.40.
 8. The process in accordance with claim 6 wherein thecondensable inert fluid is isopentane.
 9. The process in accordance withclaim 8 wherein the concentration of isopentane in said fluidizingmedium is about 3 to about 20 mole percent.
 10. The process inaccordance with claim 6 wherein the condensable inert fluid isisohexane.
 11. The process in accordance with claim 10 wherein theconcentration of isohexane in said fluidizing medium is about 1.5 toabout 10 mole percent.
 12. The process in accordance with claim 1wherein said fluidizing medium comprises:i) butene-1 and ethylene at amolar ratio of from about 0.001 to about 0.60 or 4-methyl-pentene-1 andethylene at a molar ratio of from about 0.001 to about 0.50 or hexene-1and ethylene at a molar ratio of from about 0.001 to about 0.30 oroctene-1 and ethylene at a molar ratio of from about 0.001 to about0.10; ii) a condensable inert fluid comprising from about 1.5 to about30 mole percent of the fluidizing medium.
 13. The process in accordancewith claim 12 wherein hydrogen is present at a molar ratio with respectto ethylene of from about 0.00 to about 1.6.
 14. The process inaccordance with claim 12 wherein the condensable inert fluid isisopentane.
 15. The process in accordance with claim 14 wherein theconcentration of isopentane in said fluidizing medium is about 3 toabout 30 mole percent.
 16. The process in accordance with claim 12wherein the condensable inert fluid is isohexane.
 17. The process inaccordance with claim 16 wherein the concentration of isohexane in saidfluidizing medium is about 1.5 to about 15 mole percent.
 18. The processin accordance with claim 1 wherein said fluidizing medium comprises:i)butene-1 and ethylene at a molar ratio of from about 0.001 to about 0.30or 4-methyl-pentene-1 and ethylene at a molar ratio of from about 0.001to about 0.25 or hexene-1 and ethylene at a molar ratio of from about0.001 to about 0.15 or octene-1 and ethylene at a molar ratio of fromabout 0.001 to about 0.05; ii) a condensable inert fluid comprising fromabout 5 to about 40 mole percent of the fluidizing medium.
 19. Theprocess in accordance with claim 18 wherein hydrogen is present at amolar ratio with respect to ethylene of from about 0.00 to 1.5.
 20. Theprocess in accordance with claim 18 wherein the condensable inert fluidis isopentane.
 21. The process in accordance with claim 20 wherein theconcentration of isopentane in said fluidizing medium is about 3 toabout 20 mole percent.
 22. The process in accordance with claim 18wherein the condensable inert fluid is isohexane.
 23. The process inaccordance with claim 22 wherein the concentration of isohexane in saidfluidizing medium is about 1.5 to about 15 mole percent.
 24. A processfor polymerizing alphaoolefin(s) in a gas phase reactor having afluidized bed and a fluidizing medium for producing film and moldinggrade products, the improvement comprising operating said reactor suchthat the enthalpy change of said fluidizing medium entering and exitingthe reactor is in the range of from 42 Btu/lb to 110 Btu/lb andmaintaining the ratio of fluidized bulk density to settled bulk densityabove 0.59.
 25. The process in accordance with claim 24 wherein thefluidizing medium comprises a gas phase and a liquid phase wherein thelevel of liquid entering the reactor is in the range of from 20 to 50weight percent based on the total weight of the fluidizing medium. 26.The process in accordance with claim 24 wherein the enthalpy change isin the range of between about 50 Btu/lb to about 100 Btu/lb.
 27. Theprocess in accordance with claim 24 wherein the product is withdrawn ata rate above about 500 lb/hr-ft².
 28. A continuous process forincreasing reactor productivity of a gas phase polymerization reactorhaving a fluidizing medium and a fluidized bed, said process comprisingpassing a gaseous stream comprising monomer through a reaction zone inthe presence of a catalyst to produce a polymeric product, withdrawingsaid polymeric product, withdrawing said fluidizing medium comprisingunreacted monomer from said reaction zone, mixing said fluidizing mediumwith hydrocarbon and polymerizable monomer(s) to form a liquid phase anda gas phase, and recycling said fluidizing medium to said reactor, theimprovement comprising:a) introducing said hydrocarbon into saidfluidizing medium to permit an increase in the cooling capacity of thefluidizing medium to a level in the range of from 42 Btu/lb to 110Btu/lb; b) increasing the rate of withdrawal of polymer product to above500 lb/hr-ft² ; and c) maintaining the ratio of fluidized bulk densityto settled bulk density above 0.59.
 29. The process in accordance withclaim 28 wherein the liquid phase comprises a level of liquid in therange of from 20 to 50 weight percent based on the total weight of thefluidizing medium.
 30. The process of claim 1 wherein the level ofliquid is in the range of from 20 to 50 weight percent based on thetotal weight of the fluidizing medium.
 31. The process in accordancewith claim 1 wherein the level of liquid is in the range of from 20 to44.2 weight percent based on the total weight of the fluidizing medium.32. The process in accordance with claim 1 wherein the level of liquidis in the range of from 22 to 30 weight percent based on the totalweight of the fluidizing medium.
 33. The process in accordance withclaim 1 wherein the level of liquid is in the range of from 25 weightpercent to 38.6 weight percent based on the total weight of thefluidizing medium.
 34. The process in accordance with claim 24 whereinthe enthalpy change is in the range of from 60 Btu/lb to 100 Btu/lb. 35.The process in accordance with claim 24 wherein the enthalpy change isin the range of from 73 Btu/lb to 100 Btu/lb,
 36. The process inaccordance with claim 24 wherein the level of liquid in the fluidizingmedium based on the total weight of the fluidizing medium is in therange of from 30 to 50 weight percent,
 37. The process in accordancewith claim 24 wherein the level of liquid is in the range of from 20 to44.2 weight percent based on the total weight of the recycle stream. 38.The process in accordance with claim 24 wherein the level of liquid isin the range of from 22 to 30 weight percent based on the total weightof the recycle stream.
 39. The process in accordance with claim 24wherein the level of liquid is in the range of from 25 to 38.6 weightpercent based on the total weight of the recycle stream.
 40. The processin accordance with claim 28 wherein the level of liquid in thefluidizing medium based on the total weight of the fluidizing medium isin the range of between from 30 to 50 weight percent.
 41. The process inaccordance with claim 28 wherein the level of liquid is in the range offrom 20 to 44.2 weight percent based on the total weight of the recyclestream.
 42. The process in accordance with claim 28 wherein the level ofliquid is in the range of from 22 to 30 weight percent based on thetotal weight of the recycle stream.
 43. The process in accordance withclaim 28 wherein the level of liquid is in the range of from 25 to 38.6weight percent based on the total weight of the recycle stream.